The present invention relates to methods for making porous products of thermoplastic polymers and like materials by sintering the materials, and the porous products made by the methods.
Porous products of thermoplastic polymers and the like, which are commonly called porous plastics, are structures featuring interconnected (i.e. open cell) omnidirectional pores. The size of the pores typically is in the range of 5 microns to 500 microns. The structures are in a wide variety of product configurations for wicking, venting, filtering, sparging, etc. Porous plastic products include such things as nibs, catheter vents, compressed air filters, water filters, and waste water treatment diffusers.
Porous plastics are made by a form of sintering. Sintering is the process of fusing discrete particles by heat, with or without pressure, to form a porous structure. The sintering process uses raw material in the form of discrete particles of a thermoplastic polymer or the like.
A significant problem has been that some polymers are more difficult to sinter than others. It is believed that some difficult-to-sinter polymers, such as low density polyethylene and polyurethane, might have been impossible to sinter heretofore. Furthermore, polymers are primarily sold in the form of pellets, which are typically rod or cylindrical shaped nuggets of polymer 1/8 to 1/4 inch in diameter. The pellets are too big to be sintered into structures with pore sizes suitable for most applications. The pellets can be ground to get raw materials of desired particle sizes, but such grinding is difficult and costly. The few plastics commercially available in flake or powder form constitute the bulk of materials used for sintering plastics.
The properties of some porous plastic products are not entirely satisfactory to perform the functions for which the products are used. For example, applicator heads of porous plastics, used on applicators for antiperspirants and deodorants in the form of fluent materials including creams, gels and the like, are somewhat abrasive. As a result, some users experience skin irritation. As another example, porous plastic is often used as a bubbler or diffuser to oxygenate water in wastewater treatment. Air is forced under pressure through the pores of a submerged hollow tube having a wall of porous plastic. The air exits the outer surface of the tube wall as bubbles that float up through the water. Oxygen diffuses into the water around each bubble. Traditional porous plastic bubblers have a broad range of pore sizes and thus emit a broad range of bubble sizes. However, the emission of large bubbles limits the efficiency of the gas transfer. Selective filtration is another example of an application where a broad pore size distribution is a limitation. Selective filtration refers to situations wherein one wishes to filter or exclude particles of a specific size but not exclude slightly smaller particles. This is difficult with a porous medium having a broad range of pore sizes.